The not-quite-as-good news, at least for the ad-tech guys: It looks like the Exchange is always going to be a side project for Facebook.

So says Bernstein Research analyst Carlos Kirjner, who figures that Facebook Exchange ads, which let advertisers pitch you based on your Web traffic history, will only end up accounting for 15 percent to 20 percent of Facebook’s old-fashioned Web ads. Those are the ones that run on the right side of the service’s page when you look at it on a PC.

And Kirjner figures that the Facebook Exchange will be capped when it comes to its conventional ads, too. That’s for two reasons: He says there simply may not be enough demand to turn Facebook’s huge pool of regular ads into retargeted ones. And he says Facebook will move slowly with this stuff to avoid setting off alarms with U.S. and European regulators, who are already sniffing around retargeting and privacy issues.

But even if the Facebook Exchange plays a limited role for the company, it’s still an important one. Kirjner predicts that Exchange ads will help slow the decline of Facebook’s conventional ad business, which is dropping off as users head to the mobile Web.

If the Exchange wasn’t around, he says, conventional ad pricing would drop 5 percent next year, and 5 percent the year after that. But after factoring in the growth of Facebook Exchange, which adds a big pricing premium, Kirjner figures those ads will remain flat next year, and drop a bit in 2014.

That is: Without the Exchange, this chart would show a much steeper drop.

*Kirjner does think the company’s first efforts at mobile are working, by the way, and has moved his rating from “market-perform” to “outperform” — in conventional English, that’s a “buy.”

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